1. Field of the Invention
The present invention relates in general to the fabrication of glass pipettes and relates in particular to an apparatus which forms fine tipped glass pipettes during a pulling operation and which is adapted to modify the geometry of the pipette tips during a heat treating operation.
2. Description of Prior Developments
Equipment for producing pipettes has been available in numerous configurations for producing various types of fine tipped glass tubes such as injection pipettes and patch pipettes. Although such apparatus has generally functioned satisfactorily, most require significant knowledge and skill to operate properly so as to produce uniform results and consistent product quality.
That is, conventional pipette pulling apparatus typically apply a tensile load to a thin glass tube as the tube is heated to a point where the tensile strength of the glass decreases. The tube then stretches or elongates under the applied load as the heated and softened region necks down and breaks in two. At this point, the fine tipped section created at the break is typically removed from the pulling apparatus and further conditioned in a secondary heat treating operation.
More particularly, the fine tipped pipettes are placed in a secondary heating apparatus known as a xe2x80x9cforgexe2x80x9d where the fine tips of the tubes are heated under controlled conditions to modify the geometry of the glass tips. During these heating operations, generally referred to as xe2x80x9cfire polishingxe2x80x9d, the tips of the pipettes are typically moved back and forth over a heating element or within an oven so as to modify the shape and size of the opening within and around the tip of the pipette.
It can be appreciated that the secondary forging or heating operation is not only time consuming, since it necessitates removal of the pipette from a pulling apparatus and placement in a forge, but is also expensive insofar as a separate forging apparatus must be purchased to carry out additional heating operations.
Another problem encountered with conventional pipette pulling apparatus is the relatively complex process involved in producing larger diameter patch pipettes. While small diameter injection pipettes, also known as intracellular pipettes, can be fabricated with a single pulling step, larger diameter patch pipettes typically require a two stage heating and pulling operation which is more labor intensive and subject to more process variables than a single stage pulling operation.
During a two stage pulling operation, a glass tube is initially heated at a first predetermined temperature while being subjected to a first predetermined tensile load. Once the tube elongates a predetermined amount, the tube elongation and the travel of that half of the pipette being pulled (typically by a dead weight) is limited by a preset travel stop.
At this point, the heating element, which was initially centered over the necked down region between the two halves of the heated tube, is deactivated and manually moved several millimeters to a second preset location which coincides with the new location of the center of the necked down region of the glass tube. The heater is then reactivated to a second preset temperature which is typically higher than that of the first heating step.
Once the heater is reactivated for a preset period of time, a second tensile load is applied and the travel stop is manually removed to allow the tube to stretch even further to the point where it breaks in two. The second tensile load can be applied as a simple dead weight which operates purely by gravity, or by a positively driven load produced by a solenoid. Again, once the tube is pulled apart, the tip of the pipette tube is typically removed from the pulling apparatus and heated in a separate heating apparatus, such as a forge, where a reciprocatory fire-polishing operation is effected. This entire operation requires considerable expertise, and is subject to numerous process variables leading to non-uniform pipette tip geometry.
Accordingly, a need exists for a method and apparatus for producing glass pipettes which reduces the level of operator skill required to consistently produce high quality pipettes having uniform dimensions and uniform functional characteristics.
A further need exists for such a method and apparatus which obviates the need for a separate heating apparatus or xe2x80x9cforgexe2x80x9d for carrying out secondary heating operations on a pipette, such as fire polishing operations.
Yet another need exists for such a method and apparatus which is highly automated so as to reduce the amount of labor and time required to produce high quality glass pipettes.
The aforementioned objects, features and advantages of the invention will in part, be pointed out with particularity, and will, in part, become obvious from the following more detailed description of the invention, taken in conjunction with the accompanying drawings, which formn an integral part thereof.
The present invention has been developed to fulfill the needs noted above and therefore has as an object the provision of a method and apparatus for producing fine tipped glass pipettes with relatively low skilled operators.
A further object of the invention is the provision of a method and apparatus for consistently producing high quality glass pipettes in a highly automated system which promotes uniform product features and product performance.
Still a further object of the invention is to provide a highly automated method and apparatus for producing high quality glass pipettes using a minimum amount of labor and time.
Yet a further object of the invention is to provide a method and apparatus which obviates the need for separate heating apparatus for carrying out secondary heating operations on a pipette tip, such as fire polishing operations.
Another object of the invention is the provision of a method and apparatus for producing both single stage and two stage pulled pipettes which are loaded and drawn with a highly controlled positive driver which is not limited to acceleration provided by gravity.
Yet another object of the invention is the provision of a method and apparatus for producing pipettes using a linear motor for generating highly controlled tensile loads on glass tubes as they are stretched into fine tipped pipettes.
These and other objects are met by the present invention which is directed to a method and apparatus for producing high quality glass pipettes having uniform dimensions and consistent product performance. Rather than applying a tensile load to a pipette tube with a dead weight during a pulling operation, the present invention uses a highly accurate and highly controllable linear motor to provide a carefully controlled tensile pulling load to such a glass tube as it is being heated and stretched.
Because a linear motor can apply a controlled variable stroke length and a controlled driving force in two opposing axial directions, a pipette can be reintroduced into the same heater as that used during pulling for subsequent heating as required, for example, to finely finish the geometry of the pipette tip. Moreover, because a linear motor can provide high acceleration to a pipette as it is being elongated, highly customized pipette tip configurations can be produced.
A particular advantage of a linear motor, as used in the present invention, is the ability to produce a carefully timed sequence of controlled movement, such as the reciprocatory movement often used during fire polishing. Such finely controlled variable length movements are not possible with conventional fixed stroke solenoid-actuated pipette pullers.
The aforementioned objects, features and advantages of the invention will, in part, be pointed out with particularity, and will, in part, become obvious from the following more detailed description of the invention, taken in conjunction with the accompanying drawings, which form an integral part thereof.